'\" et
.TH PTHREAD_MUTEX_LOCK "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
pthread_mutex_lock,
pthread_mutex_trylock,
pthread_mutex_unlock
\(em lock and unlock a mutex
.SH SYNOPSIS
.LP
.nf
#include <pthread.h>
.P
int pthread_mutex_lock(pthread_mutex_t *\fImutex\fP);
int pthread_mutex_trylock(pthread_mutex_t *\fImutex\fP);
int pthread_mutex_unlock(pthread_mutex_t *\fImutex\fP);
.fi
.SH DESCRIPTION
The mutex object referenced by
.IR mutex
shall be locked by a call to
\fIpthread_mutex_lock\fR()
that returns zero or
.BR [EOWNERDEAD] .
If the mutex is already locked by another thread, the calling thread
shall block until the mutex becomes available. This operation shall
return with the mutex object referenced by
.IR mutex
in the locked state with the calling thread as its owner. If a thread
attempts to relock a mutex that it has already locked,
\fIpthread_mutex_lock\fR()
shall behave as described in the
.BR Relock
column of the following table. If a thread attempts to unlock a mutex
that it has not locked or a mutex which is unlocked,
\fIpthread_mutex_unlock\fR()
shall behave as described in the
.BR "Unlock When Not Owner"
column of the following table.
.TS
center box tab(!);
cB | cB | cB | cB
l | l | l | l.
Mutex Type!Robustness!Relock!Unlock When Not Owner
_
NORMAL!non-robust!deadlock!undefined behavior
_
NORMAL!robust!deadlock!error returned
_
ERRORCHECK!either!error returned!error returned
_
RECURSIVE!either!recursive!error returned
!!(see below)
_
DEFAULT!non-robust!undefined!undefined behavior\s-2\(dg\s+2
!!behavior\s-2\(dg\s+2
_
DEFAULT!robust!undefined!error returned
!!behavior\s-2\(dg\s+2
.TE
.IP "\(dg" 6
If the mutex type is PTHREAD_MUTEX_DEFAULT, the behavior of
\fIpthread_mutex_lock\fR()
may correspond to one of the three other standard mutex types as described
in the table above. If it does not correspond to one of those three,
the behavior is undefined for the cases marked \(dg.
.P
Where the table indicates recursive behavior, the mutex shall maintain
the concept of a lock count. When a thread successfully acquires a
mutex for the first time, the lock count shall be set to one. Every
time a thread relocks this mutex, the lock count shall be incremented
by one. Each time the thread unlocks the mutex, the lock count shall be
decremented by one. When the lock count reaches zero, the mutex shall
become available for other threads to acquire.
.P
The
\fIpthread_mutex_trylock\fR()
function shall be equivalent to
\fIpthread_mutex_lock\fR(),
except that if the mutex object referenced by
.IR mutex
is currently locked (by any thread, including the current thread), the
call shall return immediately. If the mutex type is
PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the
calling thread, the mutex lock count shall be incremented by one and
the
\fIpthread_mutex_trylock\fR()
function shall immediately return success.
.P
The
\fIpthread_mutex_unlock\fR()
function shall release the mutex object referenced by
.IR mutex .
The manner in which a mutex is released is dependent upon the mutex's type
attribute. If there are threads blocked on the mutex object referenced by
.IR mutex
when
\fIpthread_mutex_unlock\fR()
is called, resulting in the mutex becoming available, the scheduling
policy shall determine which thread shall acquire the mutex.
.P
(In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become
available when the count reaches zero and the calling thread no longer
has any locks on this mutex.)
.P
If a signal is delivered to a thread waiting for a mutex, upon return
from the signal handler the thread shall resume waiting for the mutex
as if it was not interrupted.
.P
If
.IR mutex
is a robust mutex and the process containing the owning thread
terminated while holding the mutex lock, a call to
\fIpthread_mutex_lock\fR()
shall return the error value
.BR [EOWNERDEAD] .
If
.IR mutex
is a robust mutex and the owning thread terminated while holding the
mutex lock, a call to
\fIpthread_mutex_lock\fR()
may return the error value
.BR [EOWNERDEAD] 
even if the process in which the owning thread resides has not
terminated. In these cases, the mutex is locked by the thread but the
state it protects is marked as inconsistent. The application should
ensure that the state is made consistent for reuse and when that is
complete call
\fIpthread_mutex_consistent\fR().
If the application is unable to recover the state, it should unlock the
mutex without a prior call to
\fIpthread_mutex_consistent\fR(),
after which the mutex is marked permanently unusable.
.P
If
.IR mutex
does not refer to an initialized mutex object, the behavior of
\fIpthread_mutex_lock\fR(),
\fIpthread_mutex_trylock\fR(),
and
\fIpthread_mutex_unlock\fR()
is undefined.
.SH "RETURN VALUE"
If successful, the
\fIpthread_mutex_lock\fR(),
\fIpthread_mutex_trylock\fR(),
and
\fIpthread_mutex_unlock\fR()
functions shall return zero; otherwise, an error number shall be
returned to indicate the error.
.SH ERRORS
The
\fIpthread_mutex_lock\fR()
and
\fIpthread_mutex_trylock\fR()
functions shall fail if:
.TP
.BR EAGAIN
The mutex could not be acquired because the maximum number of recursive
locks for
.IR mutex
has been exceeded.
.TP
.BR EINVAL
The
.IR mutex
was created with the protocol attribute having the value
PTHREAD_PRIO_PROTECT
and the calling thread's priority is higher than the mutex's current
priority ceiling.
.TP
.BR ENOTRECOVERABLE
.br
The state protected by the mutex is not recoverable.
.TP
.BR EOWNERDEAD
.br
The mutex is a robust mutex and the process containing the previous
owning thread terminated while holding the mutex lock. The mutex lock
shall be acquired by the calling thread and it is up to the new owner
to make the state consistent.
.P
The
\fIpthread_mutex_lock\fR()
function shall fail if:
.TP
.BR EDEADLK
The mutex type is PTHREAD_MUTEX_ERRORCHECK and the current
thread already owns the mutex.
.P
The
\fIpthread_mutex_trylock\fR()
function shall fail if:
.TP
.BR EBUSY
The
.IR mutex
could not be acquired because it was already locked.
.P
The
\fIpthread_mutex_unlock\fR()
function shall fail if:
.TP
.BR EPERM
The mutex type is PTHREAD_MUTEX_ERRORCHECK or PTHREAD_MUTEX_RECURSIVE,
or the mutex is a robust mutex, and the current thread does not own
the mutex.
.br
.P
The
\fIpthread_mutex_lock\fR()
and
\fIpthread_mutex_trylock\fR()
functions may fail if:
.TP
.BR EOWNERDEAD
.br
The mutex is a robust mutex and the previous owning thread terminated
while holding the mutex lock. The mutex lock shall be acquired by the
calling thread and it is up to the new owner to make the state consistent.
.P
The
\fIpthread_mutex_lock\fR()
function may fail if:
.TP
.BR EDEADLK
A deadlock condition was detected.
.P
These functions shall not return an error code of
.BR [EINTR] .
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
Applications that have assumed that non-zero return values are errors
will need updating for use with robust mutexes, since a valid return
for a thread acquiring a mutex which is protecting a currently
inconsistent state is
.BR [EOWNERDEAD] .
Applications that do not check the error returns, due to ruling out the
possibility of such errors arising, should not use robust mutexes. If
an application is supposed to work with normal and robust mutexes it
should check all return values for error conditions and if necessary
take appropriate action.
.SH RATIONALE
Mutex objects are intended to serve as a low-level primitive from which
other thread synchronization functions can be built. As such, the
implementation of mutexes should be as efficient as possible, and this
has ramifications on the features available at the interface.
.P
The mutex functions and the particular default settings of the mutex
attributes have been motivated by the desire to not preclude fast,
inlined implementations of mutex locking and unlocking.
.P
Since most attributes only need to be checked when a thread is going to
be blocked, the use of attributes does not slow the (common)
mutex-locking case.
.P
Likewise, while being able to extract the thread ID of the owner of a
mutex might be desirable, it would require storing the current thread
ID when each mutex is locked, and this could incur unacceptable levels
of overhead. Similar arguments apply to a
.IR mutex_tryunlock
operation.
.P
For further rationale on the extended mutex types, see the Rationale (Informative) volume of POSIX.1\(hy2017,
.IR "Threads Extensions".
.P
If an implementation detects that the value specified by the
.IR mutex
argument does not refer to an initialized mutex object, it is
recommended that the function should fail and report an
.BR [EINVAL] 
error.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.ad l
.IR "\fIpthread_mutex_consistent\fR\^(\|)",
.IR "\fIpthread_mutex_destroy\fR\^(\|)",
.IR "\fIpthread_mutex_timedlock\fR\^(\|)",
.IR "\fIpthread_mutexattr_getrobust\fR\^(\|)"
.ad b
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.12" ", " "Memory Synchronization",
.IR "\fB<pthread.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
